1. Field of the Invention
The present invention relates to an optical scanning device, an image forming apparatus, and an optical communication system. In particular, the present invention relates to an optical scanning device including a vertical-cavity surface-emitting laser as a light source and used in an image forming apparatus such as a copier, a printer, a facsimile, a plotter, or a multifunction peripheral having a plurality of these functions; and an image forming apparatus and an optical communication system having the optical scanning device.
2. Description of the Related Art
In recent years, high-speed, high-density, and high-image-quality image forming apparatuses have been demanded. To meet the demand, an image forming apparatus using a multibeam writing system provided with a plurality of light-emitting points to scan a single scanned surface with a plurality of beams has been put to practical use.
Optical scanning devices, in particular, laser scanning devices using semiconductor lasers have been widely applied to image forming apparatuses such as image display devices or image recording devices because the laser scanning devices have simple structures, high-speed performance, and high resolution capability. The laser scanning devices are especially suitable as exposure devices of electrophotographic printers, and a large number of products as laser beam printers have been put in the market.
Furthermore, in more recent years, a demand for higher-speed and higher-resolution image forming apparatuses are expanding, and increase in scanning speed is also being desired. To realize the high-speed scanning, a high-speed deflecting device is necessary. However, if a rotating polygon mirror for example is used as the deflecting device, increase in the number of rotations may be limited.
As one countermeasure against the limited number of rotations, as disclosed in Japanese Patent No. 3227226 for example, there has been proposed an image forming apparatus having a so-called multibeam scanning device that performs scanning with beams from a vertical cavity surface emitting laser (VCSEL), which is a surface emitting laser having a plurality of independently-modulatable light-emitting points, to scan a plurality of scanning lines simultaneously by one scanning.
An example of a conventional optical scanning device that includes a VCSEL having the above-mentioned plurality of light-emitting areas is illustrated in FIG. 15. In FIG. 15, a plurality of laser beams are emitted from a light source 101 having the plurality of light-emitting areas and a light-emitting control unit for controlling the light-emitting points, and collimated and shaped by a coupling optical system including a coupling lens 102, an aperture 104, and a cylindrical lens 103. The plurality of laser beams are, after being coupled, deflected and reflected by a deflector 105 serving as a deflecting unit formed of a rotating polygon mirror, and then are made to scan in a main-scanning direction. Furthermore, a first scanning lens 106a and a second scanning lens 106b constituting a scanning imaging optical system focus the plurality of laser beams onto a scanned surface (photosensitive element) 108 serving as an imaging plane, i.e., a cylindrical image carrier bearing a photosensitive medium in the present example, to form a scanning line 109. In FIG. 15, 107a, 107b, and 107c respectively denote a first, a second, and a third reflecting mirrors that reflect the laser beams passed through the second scanning lens 106b. 
The image carrier is rotated about an axis of the cylinder illustrated in FIG. 15 to move the imaging plane in a direction perpendicular to the main-scanning direction to enable optical scanning and image formation.
To realize high-speed image output, it is possible to apply means for utilizing multibeam by the above-mentioned VCSEL for example. In particular, a high-speed output device is generally provided with a writing light source utilizing multibeam.
Furthermore, in relation to the problems to be solved by the present invention, which will be described later, Japanese Patent Application Laid-open No. 2008-33062 discloses a cylindrical lens whose surface on a light source side is coated with light-quantity attenuation coating so that, when an fθ lens without antireflection film is used to focus a light beam emitted from the light source onto a photosensitive element through the fθ lens, occurrence of return light (reflected light) from the fθ lens to the light source can be prevented.
Japanese Patent No. 3243013 discloses an optical scanning device having a shading correction function, in which coating made of an oxide having birefringence property is applied onto a flat lens surface on a light input side of a flat-convex cylinder lens, which has the flat surface on a light source side, or onto a lens surface on a light output side of a collimating lens, which shapes a laser beam from the light source into a parallel beam, to transform linear polarized light into practically circular polarized light.
Japanese Patent Application Laid-open No. H05-160467 discloses using a parallel plate as an optical element such that a part of a light beam emitted from a single-mode semiconductor laser is reflected multiple times between a plurality of planes and then is caused to pass and to be transmitted through the parallel plate together with a residual light beam, and also discloses that mode hopping of the semiconductor laser is detected based on multiple interference that occurs in the parallel plate.
However, because mass production results of the VCSEL light source are smaller than those of an edge emitting laser diode (LD), the VCSEL light source has a problem in that its current light quantity (light output) range is narrower than that of the edge emitting LD.
A light source mounted on the optical scanning device needs to ensure a light output range wider than a certain range because of the first to the third reasons described below.
[First Reason]
Light emitted from the light source mounted on the optical scanning device reaches a photosensitive element via a first optics (hereinafter, also referred to as “the first optical system”), a second optics (hereinafter, also referred to as “the second optical system”), a rotating polygon mirror, and a third optics (hereinafter, also referred to as “the third optical system”). When optical elements of the optical systems are mass produced, each optical element may have different transmittance and reflectance. Consequently, a ratio between optical energy output from the light source and optical energy input to the photosensitive element, i.e., light use efficiency, may vary depending on optical scanning devices. Therefore, if constant optical energy is desired on a photosensitive element in each image forming apparatus, the light output energy from the light source needs to be adjusted.
[Second Reason]
In addition to the variation described in the first reason, variation in shading characteristics due to variation in the rotating polygon mirror and the third optical system also needs to be considered. Herein, the shading characteristics is a light quantity ratio between light quantity at the center of the photosensitive element and light quantity at a periphery of the photosensitive element other than the center in light quantity distribution in the main-scanning direction.
To correct the shading characteristics, i.e., to equalize the light quantity between the center and the periphery of the photosensitive element in the main scanning direction, as illustrated in FIG. 16, light quantity of the periphery is reduced when the light quantity of the periphery is larger than that of the center in the main scanning direction, and, the light quantity of the periphery is increased when the light quantity of the periphery is smaller than that of the center in the main-scanning direction. Accordingly, as illustrated in FIG. 17, the light quantity on the photosensitive element is made constant between the center and the periphery in the main scanning direction after the shading correction is performed, and desired light quantity is acquired. It is of course possible to increase the light quantity of the center in the main-scanning direction depending on the light quantity of the periphery. However, the above method is described only by way of example of the shading correction, and other methods are not described herein.
To perform the shading correction, it is necessary to adjust the light output energy from the light source to increase or decrease the light quantity of the periphery.
That is, as illustrated in FIG. 18, it is necessary to widen the light output range of the light source in accordance with the variation in the light use efficiency of the center of the photosensitive element and the variation in the light use efficiency of the periphery of the photosensitive element.
[Third Reason]
The optical energy from the light source in the assembled image forming apparatus is adjusted in accordance with the variation in the light use efficiencies of the photosensitive element and the optical scanning device because of the first and the second reasons. However, when the image forming apparatus is operated for a long period of time, desired optical energy quantity may be changed because of degradation of the photosensitive element or change in surrounding conditions. Therefore, the light source also needs to cope with such temporal changes.
A method that may save a situation where the light source does not satisfy the light output range desired in the first to the third reasons has been proposed. Specifically, the method is to reduce the variation in the light use efficiency between the optical scanning devices by using a neutral density filter (hereinafter, referred to as “ND filter”). In the method, when four optical scanning devices for example have different light use efficiencies of 1.05, 1.03, 1.00, and 0.97, respectively, if ND filters with transmittance of 1/1.05 and 1/1.03 are applied onto the optical scanning devices having the light use efficiencies of over 1.00, the variation in the light use efficiency can be reduced from a range from 0.97 to 1.05 to a range from 0.97 to 1.00.
As described above, the ND filter serves as effective means for reducing the variation in the light use efficiency of the writing optical system to compensate for the inadequacy of the light output range of the VCSEL light source. However, the ND filter is incompatible with the VCSEL light source, and causes the following problems.
That is, because a glass plate or a plastic plate is generally used as a substrate of the ND filter, when, for example, an ND filter having a first surface (hereinafter, also referred to as “a first plane”) and a second surface (hereinafter, also referred to as “a second plane”) that are flat glass plates parallel to each other is placed between a coupling lens and a cylindrical lens of the conventional optical scanning device as illustrated in FIG. 15, and if emission light from the coupling lens is a parallel beam (emission light from a coupling lens is usually a parallel beam in typical optical scanning devices), the parallel beam input to an ND filter 200 causes multiple reflection and multiple interference in the glass substrate having a refractive index n.
In Equations (1) and (2) described in FIG. 19, d represents a thickness of the substrate, θ represents a refracting angle, λ represents a wavelength of laser light, and an n(BC+CD)-BE represents an optical path difference between adjacent transmitted lights.
The VCSEL has an oscillation spectrum at a more preferable single wavelength than that of an edge emitting type LD (see, for example, “Photonics”, Ohmsha, Ltd., issued in December, 2007). However, the wavelength may be changed slightly in a range smaller than 1 nm as input current to the VCSEL is increased. If the wavelength is slightly changed, a phase of light emitted from each ND filter is changed, so that level of construction or destruction of multiply-reflected lights is changed. Therefore, linear-function-like linearity of the light intensity of the emission light from the ND filter is not maintained against the input current.
This phenomenon is described with reference to FIG. 19. If the wavelength λ is slightly changed due to the input current, the refractive index n and the refracting angle θ of the ND filter 200 are slightly changed, so that the level of construction or destruction of the transmitted light is changed.
As described above, regarding the technology related to the conventional optical scanning device, the following problems are not suggested and described in documents or literatures including the above-mentioned patent documents or nonpatent literature to the best knowledge of the inventors of the present invention. That is, when the light output range is not ensured because of the above-mentioned first to third reasons, and if the ND filter is arranged for adjusting and reducing the variation in the light use efficiency between the optical scanning devices, light intensity on the photosensitive element is changed according to the input current to the VCSEL because the multiple interference of the laser beam from the VCSEL occurs in the ND filter.